Fluids whose viscosity can be varied by the application of an external voltage can be used, for example, for power transfer, for shock absorption, and as valves, and as a result such fluids have recently been the subject of some scrutiny. Fluids whose viscosity increases in response to an electrical field are collectively known as electroviscous fluids. However, in order to be able to withstand highly practical applications in, for example, clutches, engine mounts, and shock absorbers, an electroviscous fluid is required whose yield value is substantially increased by the application of small voltages.
Various types of such fluids have already been proposed. These have generally taken the form, for example, of dispersions of porous inorganic particles (e.g., silica, alumina, talc, etc.) in electrical insulating fluids. Due to the formation of an electric double layer due in every case to water adsorbed on the particle surface, the particles orient in response to an external electrical field and the viscosity increases (more specifically, the fluid converts into a Bingham fluid and exhibits a yield value). This effect is hereinafter referred to as the Winslow effect. Based on the substantial practical advantages offered by silica (ease of industrial acquisition, rich potential for improvements and modifications in quality, etc.), many electroviscous fluids have been proposed that use silica in the system, for example U.S. Pat. No. 3,047,507 and Japanese Patent Application Laid Open Number Sho 61-44998 [44,998/1986]. However, these particular electroviscous fluids do not perform satisfactorily in industrial applications because they abrade the surrounding equipment, suffer from particle sedimentation, and exhibit a Winslow effect of modest degree.
In order to improve upon these deficiencies, electroviscous fluids have been proposed that comprise the dispersion of a polyelectrolyte in an electrical insulating fluid. The term polyelectrolyte collectively denotes polymeric compounds that contain ion pairs within the structure. Many natural and synthetic polyelectrolytes are known, and the ion-exchange resins are the best known. For example, Japanese Patent Application Laid Open Number Hei 1-180238 [180,238/1989] discloses an electroviscous fluid that comprises the dispersion in an electrical insulating fluid of microparticles of a polyelectrolyte that contains amine salt structures. Japanese Patent Application Laid Open Number Hei 1-262942 [262,942/1989] discloses an electroviscous fluid that comprises the dispersion in an electrical insulating fluid of particles prepared by pulverizing ion-exchange resin. The following advantages are associated with the use of such polyelectrolyte particles: because the particles are made of organic polymer, (a) they have a small specific gravity and thus resist sedimentation and (b) they have little abrasiveness for surrounding equipment; also, (c) they provide a relatively large Winslow effect. Another advantage associated with synthetic polyelectrolytes is that the particle can be freely engineered.
However, the preparation of polyelectrolyte particle-based electroviscous fluids as known in the art involves solidification of the electrolyte through three-dimensional crosslinking by some methodology followed by the preparation of microparticles by, for example, pulverization, etc. In this approach, the three- dimensional configuration of the electrolyte is locked in at the point of synthesis and the electrolyte can then no longer be reworked. Another deficiency in this approach is that the microparticulation process cannot produce perfectly spherical particles, and this in turn precludes both a satisfactory dispersion stability and a satisfactory Winslow effect. In addition, all of the polyelectrolyte particles proposed to date are carbon-based. Silicone oil as described below is the best electrical insulating fluid; however, carbon-based particles have a poor affinity for silicone oil.
Thus, as discussed above, none of the electroviscous fluids proposed to date perform satisfactorily. It is an object of the instant invention an electroviscous fluid that provides a high electroviscosity effect (yield value), has a good dispersion stability, is heat stable even at elevated temperatures of 100.degree. C. and above, and does not abrade or wear the surrounding equipment.